Conventionally, an air-fuel ratio control apparatus has been widely known, which comprises a three-way catalytic converter disposed in an exhaust passage (exhaust gas passage) of an internal combustion engine, and an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor disposed, in the exhaust passage, upstream and downstream of the three-way catalytic converter, respectively. The air-fuel ratio control apparatus performs a feedback control on an air-fuel ratio (an air-fuel ratio of the engine) of a mixture supplied to the engine based on the output value of the upstream air-fuel ratio sensor and the output value of the downstream air-fuel ratio sensor in such a manner that the air-fuel ratio of the engine coincides with (becomes equal to) a stoichiometric air-fuel ratio.
This type of air-fuel ratio control apparatus controls the air-fuel ratio of the engine utilizing a control amount (an air-fuel ratio feedback amount) common to all of the cylinders. That is, the air-fuel ratio feedback control is performed in such a manner that an average (value) of the air-fuel ratio of the mixture supplied to the entire engine coincides with the stoichiometric air-fuel ratio.
For example, when a measured value or an estimated value of an intake air amount of the engine deviates from “a true intake air amount”, each of the air-fuel ratios of each of the cylinders deviates from the stoichiometric air-fuel ratio toward “a rich side or a lean side” with respect to the stoichiometric air-fuel ratio without exception. In this case, the conventional air-fuel ratio control changes the air-fuel ratio of the air-fuel mixture supplied to the engine to “a leaner side or a richer side”. Consequently, the air-fuel mixture supplied to each of the cylinders is adjusted so as to be in the vicinity of the stoichiometric air-fuel ratio. Accordingly, a combustion in each of the cylinders comes close to a perfect combustion (a combustion occurring when the air-fuel ratio of the mixture is equal to the stoichiometric air-fuel ratio), and an air-fuel ratio of an exhaust gas flowing into the three-way catalytic converter coincides with the stoichiometric air-fuel ratio or with an air-fuel ratio close to the stoichiometric air-fuel ratio. As a result, a deterioration of emission can be avoided.
Meanwhile, an electronic control fuel injection type internal combustion engine typically comprises one fuel injector in each of the cylinders or in each of intake ports, each communicating with each of the cylinders. Accordingly, when a property (characteristic) of the injector for a specific cylinder becomes “a property that the injector injects fuel in an amount larger (more excessive) than an instructed fuel injection amount”, only an air-fuel ratio (air-fuel-ratio-of-the-specific-cylinder) of an air-fuel mixture supplied to the specific cylinder shifts to an extremely richer side. That is, a non-uniformity among air-fuel ratios of the cylinders (a variation in air-fuel ratios among the cylinders, air-fuel ratio imbalance among the cylinders) becomes high (prominent). In other words, there arises an imbalance among air-fuel ratios, each of which is an air-fuel ratio of a mixture supplied to each of a plurality of the cylinders (i.e., air-fuel ratios of individual cylinders).
In this case, the average of the air-fuel ratios of the mixtures supplied to the engine becomes an air-fuel ratio richer (smaller) than the stoichiometric air-fuel ratio. Accordingly, the feedback amount commonly used to all of the cylinders causes the air-fuel ratio of the specific cylinder to shift to a leaner (larger) air-fuel ratio so that the air-fuel ratio of the specific cylinder is made closer to the stoichiometric air-fuel ratio. However, the air-fuel ratio of the specific cylinder is still considerably richer (smaller) than the stoichiometric air-fuel ratio. Further, each of the air-fuel ratios of the other cylinders is caused to shift to a leaner (larger) air-fuel ratio so that the air-fuel ratios of the other cylinders are caused to deviate more from the stoichiometric air-fuel ratio. At this time, since the number of the other cylinders is larger than the number (which is one) of the specific cylinder, each of the air-fuel ratios of the other cylinders is caused to change to an air-fuel ratio slightly leaner (larger) than the stoichiometric air-fuel ratio. As a result, the average of the air-fuel ratios of the entire mixtures supplied to the engine is caused to become roughly equal to the stoichiometric air-fuel ratio.
However, the air-fuel ratio of the specific cylinder is still richer (smaller) than the stoichiometric air-fuel ratio, and the air-fuel ratios of the other cylinders are still leaner (larger) than the stoichiometric air-fuel ratio, and therefore, a combustion condition of the mixture in each of the cylinders is different from the perfect combustion condition. As a result, an amount of emissions (an amount of unburnt substances and/or an amount of nitrogen oxides) discharged from each of the cylinders increases. Accordingly, even though the average of the air-fuel ratios of the mixtures supplied to the engine coincides with the stoichiometric air-fuel ratio, the three-way catalytic converter may not be able to purify the increased emissions, and thus, there is a possibility that the emissions become worse. It is therefore important to detect whether or not the air-fuel ratio non-uniformity among cylinders becomes excessively large, since an appropriate measure can be taken in order not to worsen the emissions.
One of such conventional apparatuses (monitoring apparatuses) that determine “whether or not the non-uniformity of the air-fuel ratios among cylinders (the air-fuel ratio imbalance among cylinders, an imbalance among air-fuel ratios of individual cylinders) becomes excessively large” obtains an estimated air-fuel ratio representing each of the air-fuel ratios of each of the cylinders by analyzing an output of a single air-fuel ratio sensor disposed at an exhaust gas aggregated portion. The conventional apparatus determines whether or not “the non-uniformity of the air-fuel ratios among cylinders” becomes excessively large based on the estimated air-fuel ratio of each of the cylinders (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-220489).